Variable cylinder capacity engine

ABSTRACT

The invention concerns a mechanical transmission device for a variable cylinder capacity engine comprising at least a cylinder ( 10 ) wherein moves a piston ( 2 ), which is integral in its lower portion with a transmission member ( 3 ) co-operating through a small-sized rack ( 37 ) with a roller bearing guide device ( 4 ) and through another large-sized rack ( 35 ) with a sprocket wheel ( 5 ) linked to a connecting rod ( 6 ), a control member ( 7 ) comprising a large-sized rack ( 73 ) co-operating with the sprocket wheel ( 5 ), means for fixing ( 24, 31 ) the piston ( 2 ) on the transmission member ( 3 ) which provide a preload for clamping it and ensure its centring, linking means ( 11, 12; 71, 72 ) for rigidifying the transmission ( 3 ) and control ( 7 ) members and for increasing the flexural strength of the teeth of the large-sized racks ( 35, 73 ), and means for lightening ( 56 ) the sprocket wheel ( 5 ) which maintain its rigidity and load capacity.

[0001] The present invention relates to a mechanical transmission devicefor guiding the piston or pistons of a variable cylinder capacityengine.

[0002] The mechanical transmission device according to the presentinvention consists in improving the transmission device described andprotected in French patent application No. 98 15089 belonging to theApplicant.

[0003] Specifically, the transmission device described in patent No. 9815089 required the solution of problems associated with its manufacturein terms of the weight, rigidity and load-bearing capability of thevarious components, the risks of the piston becoming seized in thecylinder and the vertical compactness of the engine as a whole.

[0004] To do that, the mechanical transmission device according to thepresent invention proposes solutions for lightening the racks and thetoothed wheel while at the same time preserving their rigidity, forimproving the guidance and fixing of the piston and for reducing thevertical bulk of the mechanical transmission device.

[0005] The mechanical transmission device according to the presentinvention addresses the risks of seizure of the piston by, on the onehand, guiding said piston in its cylinder and, on the other hand,providing guidance with respect to the engine block of the transmissionmember secured to the piston using a synchronized rolling guide device.

[0006] The mechanical transmission device according to the presentinvention is designed in such a way that it exhibits reduced verticalbulk.

[0007] The mechanical transmission device according to the presentinvention has the advantage of making it easier to machine eachcomponent of the variable cylinder capacity engine.

[0008] The mechanical transmission device for a variable cylindercapacity engine according to the present invention comprises at leastone cylinder in which there moves a piston which is secured at its lowerpart to a transmission member collaborating, on the one hand, by meansof a small-sized rack with a rolling guide device and, on the otherhand, by means of another, large-sized rack, with a toothed wheelconnected to a connecting rod, a control member comprising a large-sizedrack collaborating with the toothed wheel, fixing means for fixing thepiston onto the transmission member which offer clamping preload andcenter it, connecting means which stiffen the transmission and/orcontrol members and increase the bending strength of the teeth of thelarge-sized racks, and lightening means for lightening the toothed wheeland which preserve its rigidity and ability to transmit force.

[0009] The mechanical transmission device according to the presentinvention comprises means for guiding the piston which consist, at thecylinder of a short guide skirt secured to the piston and, at thetransmission member, of a rolling guide device which comprises a singlesynchronized roller.

[0010] The mechanical transmission device according to the presentinvention comprises means for fixing the piston onto the transmissionmember which consist of a threaded bore produced in a supporting leg ofthe piston and of a vertical threaded rod secured to the transmissionmember, which collaborates with the threaded bore to obtain a fixingthat offers preload in the clamping of the piston onto the transmissionmember.

[0011] The mechanical transmission device according to the presentinvention comprises a piston of which the threaded bore comprises aplain coaxial bore collaborating with a plain portion provided on thevertical rod of the transmission member so as to center the piston as itis fixed onto the transmission member.

[0012] The mechanical transmission device according to the presentinvention comprises a piston of which the supporting leg has, in itsinternal part, a threaded bore communicating with radial openings whichopen into the upper part of the supporting leg.

[0013] The mechanical transmission device according to the presentinvention comprises a piston of which the radial openings open betweenfins provided on the external periphery of the supporting leg.

[0014] The mechanical transmission device according to the presentinvention comprises a piston provided, between the guide skirt and apiston ring groove, ducts which open between the fins of the supportingleg.

[0015] The mechanical transmission device according to the presentinvention comprises connecting means connecting the teeth of thelarge-sized rack of the transmission member which consist of lateralcheeks stiffening the transmission member and increasing the bendingstrength of the teeth of the large-sized rack.

[0016] The mechanical transmission device according to the presentinvention comprises a transmission member of which the lateral cheekscan be used as runway tracks collaborating with runway tracks positionedlaterally on the toothed wheel.

[0017] The mechanical transmission device according to the presentinvention comprises connecting means connecting the teeth of thelarge-sized rack of the control member which consist of lateral cheeksstiffening the control member and increasing the bending strength of theteeth of the large-sized rack.

[0018] The mechanical transmission device according to the presentinvention comprises a control member of which the lateral cheeks can beused as runway tracks collaborating with runway tracks positionedlaterally on the toothed wheel.

[0019] The mechanical transmission device according to the presentinvention comprises transmission and control members in which thelarge-sized racks comprise lateral cheeks which are produced bymachining and/or forging the bodies that make up said transmission andcontrol members.

[0020] The mechanical transmission device according to the presentinvention comprises transmission and control members in which thelarge-sized racks comprise lateral cheeks which are added onto and fixedto bodies that make up said transmission and control members.

[0021] The mechanical transmission device according to the presentinvention comprises a toothed wheel exhibiting a cutout on its upperprofile so as to reduce the vertical bulk of the engine.

[0022] The mechanical transmission device according to the presentinvention comprises a toothed wheel which is made up of two toothedhalf-wheels joined together.

[0023] The mechanical transmission device according to the presentinvention comprises a toothed wheel which is made up of two toothedhalf-wheels joined together exhibiting a cutout on their upper profileso as to reduce the vertical bulk of the engine.

[0024] The mechanical transmission device according to the presentinvention comprises lightening means for lightening the toothed wheelwhich consist of at least one cavity.

[0025] The mechanical transmission device according to the presentinvention comprises a toothed wheel and large-sized racks belonging tothe transmission and control members, the teeth of which have differentwidths.

[0026] The mechanical transmission device according to the presentinvention comprises a toothed wheel and large-sized racks belonging tothe transmission and control members, the teeth of which are ofdifferent dimensions.

[0027] The mechanical transmission device according to the presentinvention comprises a toothed wheel and large-sized racks belonging tothe transmission and control members, the teeth of which have differentprofiles.

[0028] The mechanical transmission device according to the presentinvention comprises a toothed wheel the teeth of which exhibitimpressions on their lateral faces which determine the stiffness of theedge of these teeth.

[0029] The mechanical transmission device according to the presentinvention comprises a toothed wheel of which the internal faces of thecentral cavity exhibit guide surfaces for the lateral guidance of thesmall end of the connecting rod.

[0030] The mechanical transmission device according to the presentinvention comprises a toothed wheel of which the internal faces of eachtoothed half-wheel exhibit a guide surface for the lateral guidance ofthe small end of the connecting rod.

[0031] The mechanical transmission device according to the presentinvention comprises a toothed wheel of which the pivot axis situated ina bore is offset from the center of the pitch circle of the toothedwheel so as to obtain variations in the kinematics of the piston.

[0032] The mechanical transmission device according to the presentinvention comprises a transmission member of which the threaded rodexhibits, at its middle, a hole which opens, on the one hand, above arunway track of the transmission member and, on the other hand, into thethreaded bore formed in the supporting leg of the piston.

[0033] The mechanical transmission device according to the presentinvention comprises transmission and control members the runway tracksof which are produced from the machining and/or forging of the bodies ofwhich said transmission and control members are made.

[0034] The mechanical transmission device according to the presentinvention comprises a transmission member which is provided withlightening means.

[0035] The mechanical transmission device according to the presentinvention comprises a rolling guide device which is made up of asynchronized roller and of a small-sized rack device secured to theengine block which collaborates with the small-sized rack of thetransmission member so as, on the one hand, to guide the transmissionmember on the engine block and, on the other hand, to synchronize thevertical movement of said roller with respect to the piston.

[0036] The mechanical transmission device according to the presentinvention comprises a rolling guide device of which the roller consistsof a cylindrical body comprising, at each end, pinions and an axialguide groove provided between the pinions.

[0037] The mechanical transmission device according to the presentinvention comprises a rack device secured to the engine block and whichis made up of a central body forming a runway track comprising, at itsmiddle, a vertical rib which collaborates with the guide groove of theroller so as to provide lateral guidance therefor, and of racks whichare fixed onto the vertical edges of said body to collaborate with thepinions of the roller so as to synchronize the vertical movement of saidroller with that of the piston.

[0038] The description which will follow with reference to the appendeddrawings, given by way of nonlimiting examples, will allow for a betterunderstanding of the invention, of the features it offers and of theadvantages it is able to afford:

[0039]FIG. 1 is a perspective view illustrating all of the componentsthat make up the mechanical transmission device according to the presentinvention.

[0040]FIG. 2 is an exploded perspective view depicting the mechanicaltransmission device according to the present invention.

[0041] FIGS. 3 to 8 are views depicting in detail the piston of themechanical transmission device according to the present invention.

[0042] FIGS. 9 to 13 are views illustrating in detail the transmissionmember secured to the piston of the mechanical transmission deviceaccording to the present invention.

[0043]FIG. 14 is a perspective view showing in detail part of therolling guide device according to the present invention.

[0044] FIGS. 15 to 18 are views depicting a toothed wheel of themechanical transmission device according to the present invention.

[0045]FIG. 19 is a perspective view showing in detail the control membercollaborating with a toothed wheel of the mechanical transmission deviceaccording to the present invention.

[0046]FIGS. 1 and 2 depict a mechanical transmission device 1 for avariable cylinder capacity engine comprising a piston 2 which is guidedand driven in translational movement in a cylinder 10 of the engineblock.

[0047] The piston 2 is secured at its lower part to a transmissionmember 3 which collaborates, on the one hand, with a rolling guidedevice 4 and, on the other hand, with a toothed wheel 5.

[0048] The toothed wheel 5 transmits the motion between a crankshaft 9and the transmission member 3 secured to the piston 2 via a connectingrod 6 on which it is mounted, by means of a connecting pin 61 whichallows it to pivot.

[0049] The toothed wheel 5 collaborates at the opposite end of thetransmission member 3 with another member known as the control member 7which is mounted in a recess or on a guide which are formed in theengine block.

[0050] The position of the control member 7 with respect to the engineblock is afforded by a positioning device, not depicted, but describedin a French patent application No 98 04601 belonging to the Applicant.

[0051] FIGS. 3 to 8 show the piston 2 which is made up of a thick disk20 provided with the grooves able respectively to receive the firingring, compression ring and oil-control ring all known per se.

[0052] The disk 20 comprises, in its lower part, a short guide skirt 21which guides the piston 2 in the cylinder 10.

[0053] The underside of the thick disk 20 is secured to a supporting leg22 bearing fins 23 on its external periphery.

[0054] The supporting leg 22 comprises, in its internal part, a bore 24which opens into radial openings 25 formed in the upper part of saidsupporting leg.

[0055] The radial opening 25 open between the fins 23 of the supportingleg 22.

[0056] Between the guide skirt 21 and a piston ring groove 27 the piston2 comprises ducts 26 which open between the fins 23 of the supportingleg 22.

[0057] The bore 24 exhibits, at the base of the supporting leg 22, athreaded part 28 and a plain coaxial centering bore 29.

[0058] The supporting leg 22 comprises, on its external periphery andunderneath the fins 23, at least one purchase for a tool to turn thepiston 2 when it is being fixed onto the transmission member 3,something which, for example, takes the form of two diametricallyopposed impressions 8.

[0059] It may be noted that the piston 2 can be fixed in its unfinishedstate onto the control member 3 [sic] and then machined to form all ofthe surfaces, particularly those which have a functional surface finish.

[0060] FIGS. 9 to 13 depict the transmission member 3 which is reducedin a 1-piece body 30 comprising, in its upper part, a threaded rod 31extending vertically and collaborating with the threaded part 28 of thebore 24 when the piston 2 is attached.

[0061] The threaded rod 31 comprises a plain portion 32 whichcollaborates with the plain coaxial bore 29 of the threaded bore 24 tocenter the piston 2 on the transmission member 3.

[0062] The transmission member 3 comprises a large-sized rack 35 whichis made up of parallel teeth 34 and a runway track 36 which is set backfrom the crests of said teeth and lies near the pitch circle of thelarge-sized rack 35.

[0063] The parallel teeth 34 of the large-sized rack 35 are verticallyseparated by the runway track 36.

[0064] The transmission member 3 comprises connecting means connectingthe teeth 34 of the large-sized rack 35 which consist of lateral cheeks11 and 12 stiffening the transmission member 3 and increasing thebending strength of the teeth 34 of the large-sized rack 35.

[0065] The lateral cheeks 11, 12 and the runway track 36 are producedfrom the machining and/or forging of the body 30 of which thetransmission member 3 is made.

[0066] Provision may be made for the lateral cheeks 11 and 12 to be ableto be added onto and fixed to the body 30 of the transmission member 3.

[0067] By way of an alternative, the lateral cheeks 11 and 12 may beused as a runway track collaborating with runway tracks positionedlaterally on the toothed wheel 5. In this case, the runway track 36 maybe omitted.

[0068] The threaded rod 31 comprises, at its middle, a hole 33 whichopens, on the one hand, into the bore 24 of the piston 2 and, on theother hand, above the runway track 36 separating the teeth 34 of thelarge-sized rack 35.

[0069] The hole 33, when the engine is running, plays a part inlubricating the runway track 36 because it communicates directly withthe internal bore 24 of the piston 2 which, via the radial openings 25,receives oil from the lower part of the engine.

[0070] What really happens is that the engine lubricating oil isinjected by nozzles, not depicted, or splashed onto the underside of thepiston 2.

[0071] Thus, the oil circulates within the bore 24 of the piston 2 andbetween the fins 23 via the radial openings 25.

[0072] The transmission member 3 comprises, at the opposite end to thelarge-sized rack 35 and on one of its vertical edges 39, another,small-sized rack 37 and a runway track 38 which collaborate with therolling guide device 4 secured to the engine block.

[0073] The small-sized rack 37 may be machined directly into the body 30or added onto and fixed to the latter. A rack 37 may equally be providedon each of the vertical edges 39 of the body 30, depending on theconstruction of the rolling guide device 4.

[0074] The body 30 of the transmission member 3 is provided with alightening means which, for example, consist of a series of blindlightening holes 13.

[0075]FIGS. 2 and 14 illustrate the rolling guide device 4 whichconsists of a synchronized roller 40 and of a rack device 41 secured tothe engine block, so as, on the one hand, to guide the transmissionmember 3 on the engine block and, on the other hand, to synchronize thevertical movement of said roller with respect to the piston 2.

[0076] The roller 40 is made up of a cylindrical body 42 comprisingpinions 44 at each end.

[0077] The cylindrical body 42 at its middle and between the pinions 44has a guide groove 43.

[0078] The rack device 41 is made up of a central body 45 and of racks46 which are fixed to the vertical edges 47 of said body. The body 45comprises a runway track 48 and a vertical rib 49.

[0079] The first pinion 44 of the roller 40 collaborates, on the onehand, with the small-sized rack 37 of the transmission member 3 and, onthe other hand, with one of the racks 46 of the rack device 41.

[0080] The second pinion 44 of the roller 40 collaborates only with theother rack 46 of the device 41, while the vertical rib 49 of the body 45collaborates with the guide groove 43 of the body 42 to provide lateralguidance for the roller 40.

[0081] The body 42 of the roller 40 provides guidance for thetransmission member 3 by rolling, on the one hand, along the runwaytrack 38 of said member and, on the other hand, along the track 48 ofthe body 45 of the rack device 41.

[0082]FIGS. 1, 15 and 16 depict the toothed wheel 5 of the mechanicaltransmission device 1 according to the present invention.

[0083] The toothed wheel 5 is made up of two toothed half-wheels A and Bwhich are joined together, for example, by welding, brazing, electronwelding, laser welding or electromagnetic induction brazing.

[0084] The toothed wheel 5, which may or may not be made up of twotoothed half-wheels A and B, has a cutout 16 on its upper profile 59 toreduce the vertical bulk of the transmission device according to theinvention as a whole.

[0085] The toothed wheel 5 also exhibits a central cavity 53 into whichthere opens a bore 50 for the placement of the small end 60 of theconnecting rod 6 via the connecting pin 61.

[0086] The internal faces of the central cavity 53 of the toothed wheel5 exhibit guide surfaces 14 for the lateral guidance of the small end 60of the connecting rod 6.

[0087] Also, the internal faces of each toothed half-wheel A and B ofthe toothed wheel 5 exhibit a guide surface 14 for the lateral guidanceof the small end 60 of the connecting rod 6.

[0088] The toothed wheel 5 comprises a first set of teeth 51collaborating with the teeth 34 of the large-sized rack 35 of thetransmission member 3 secured to the piston 2 and a second set of teeth52 collaborating with the large-sized rack 73 of the control member 7.

[0089] Each set of teeth 51, 52 comprises at its middle a groove 54coaxial with the pitch circle of the toothed wheel 5.

[0090] Within each groove 54 is fixed a runway strip 55 exhibiting acurved profile similar to that of said groove. Each runway strip 55 isfixed, for example by laser welding or electron welding, into thecorresponding curved groove 54.

[0091] The toothed wheel 5 has cavities 56 which may or may not be blindin the central cavity 53. The cavities 56 are designed to lighten thetoothed wheel 5 while at the same time preserving its rigidity and itsload-bearing capacity.

[0092] The cavities 56 may be separated from one another by thin webs 57so as to stiffen the structure of the toothed wheel 5.

[0093] The set of teeth 51 comprises in its upper part teeth 58 of awidth that is reduced by comparison with the other teeth of the toothedwheel 5.

[0094] The upper cutout 16 of the toothed wheel 5 and the teeth 58 makeit possible to reduce the vertical bulk of the mechanical transmissiondevice 1.

[0095] This is because, with this configuration, the teeth 58 penetratefurther inside the cylinder 10 when the piston 2 is near the top of itsstroke.

[0096] Also, the set of teeth 52 comprises, in its lower part, teeth 58of a width which is reduced by comparison with the other teeth so as tobring the center of gravity of the toothed wheel 5 closer to its pivotaxis.

[0097] The pivot axis of the toothed wheel 5, which axis is situated inthe bore 50, may be offset from the center of the pitch circle of saidtoothed wheel. The offsetting of the pivot axis of the toothed wheel 5makes it possible to obtain variations in the kinematics of the piston2.

[0098]FIGS. 17 and 18 depict the toothed wheel 5 provided with means formastering the edge effect of the sets of teeth.

[0099] It is known that in order to avoid excessive stress on the edgesof the sets of teeth the teeth may be corrected in a way currentlytermed “curving”.

[0100] This operation which is expensive in industry may be replaced bycontrol over the stiffness of the teeth of the toothed wheel 5 acrosstheir width, and particularly on the edges of said teeth.

[0101] This control, which is the subject of the present invention, willbe obtained by making at least one impression 15 on each side of theteeth of the toothed wheel 5.

[0102] These impressions 15 have shapes such that they reduce thestiffness of the edges of the teeth of the toothed wheel 5 so as tolimit the stresses due to the edge effect.

[0103] These impressions 15 may be produced when the toothed wheel 5 isbeing forged.

[0104]FIG. 19 shows the control member 7 which comprises a large-sizedrack 73 and a vertical runway track 70 with which the runway strip 55provided at the middle of the set of teeth 52 of the toothed wheel 5collaborates.

[0105] The parallel teeth 74 of the large-sized rack 73 are verticallyseparated by the runway track 70.

[0106] The control member 7 comprises connecting means connecting theteeth 74 of the large-sized rack 73 which consist of lateral cheeks 71and 72 stiffening the control member 7 and increasing the bendingstrength of the teeth of the large-sized rack 73.

[0107] The lateral cheeks 71, 72 and the runway track 70 are producedfrom the machining and/or the forging of the body of which the controlmember 7 is made.

[0108] Provision may be made for the lateral cheeks 71 and 72 to beadded onto and fixed to the body of the control member 7.

[0109] By way of an alternative, the lateral cheeks 71 and 72 may beused as a runway track collaborating with runway tracks positionedlaterally on the toothed wheel 5. In this case, the runway track 70 maybe omitted.

[0110] It must also be understood that the foregoing description hasbeen given merely by way of example and that it does not in any wayrestrict the scope of the invention which would not be exceeded ifembodiment details described were replaced by any other equivalent.

1. A mechanical transmission device for a variable cylinder capacityengine comprising at least one cylinder (10) in which there moves apiston (2) which is secured at its lower part to a transmission member(3) collaborating, on the one hand, by means of a small-sized rack (37)with a rolling guide device (4) and, on the other hand, by means ofanother, large-sized rack (35), with a toothed wheel (5) connected to aconnecting rod (6), a control member (7) comprising a large-sized rack(73) collaborating with the toothed wheel (5), characterized in that itcomprises: fixing means (24, 31) for fixing the piston (2) onto thetransmission member (3) which offer clamping preload and center it,connecting means (11, 12; 71, 72) which stiffen the transmission (3)and/or control (7) members and increase the bending strength of theteeth of the large-sized racks (35, 73), and lightening means (56) forlightening the toothed wheel (5) and which preserve its rigidity andload-bearing capacity.
 2. The mechanical transmission device as claimedin claim 1, characterized in that it comprises means for guiding thepiston (2) which consist, at the cylinder (10) of a short guide skirt(21) secured to the piston (2) and, at the transmission member (3), of arolling guide device (4) which comprises a single synchronized roller(40).
 3. The mechanical transmission device as claimed in claim 1,characterized in that it comprises means for fixing the piston (2) ontothe transmission member (3) which consist of a threaded bore (24)produced in a supporting leg (22) of the piston (2) and of a verticalthreaded rod (31) secured to the transmission member (3), whichcollaborates with the threaded bore (24) to obtain a fixing that offerspreload in the clamping of the piston (2) onto the transmission member(3).
 4. The mechanical transmission device as claimed in claim 3,characterized in that it comprises a piston (2) of which the threadedbore (24) comprises a plain coaxial bore (29) collaborating with a plainportion (32) provided on the vertical rod (31) of the transmissionmember (3) so as to center the piston (2) as it is fixed onto thetransmission member (3).
 5. The mechanical transmission device asclaimed in claim 3, characterized in that the supporting leg (22) of thepiston (2) has, in its internal part, a threaded bore (24) communicatingwith radial openings (25) which open into the upper part of saidsupporting leg.
 6. The mechanical transmission device as claimed inclaim 5, characterized in that the radial openings (25) in the piston(2) open between fins (23) provided on the external periphery of thesupporting leg (22).
 7. The mechanical transmission device as claimed inclaim 2, characterized in that the piston (2) comprises, between theguide skirt (21) and a piston ring groove (27), ducts (26) which openbetween the fins (23) of the supporting leg (22).
 8. The mechanicaltransmission device as claimed in claim 1, characterized in that itcomprises connecting means connecting the teeth (34) of the large-sizedrack (35) of the transmission member (3) which consist of lateral cheeks(11, 12) stiffening the transmission member (3) and increasing thebending strength of the teeth (34) of the large-sized rack (35).
 9. Themechanical transmission device as claimed in claim 8, characterized inthat the lateral cheeks (11, 12) can be used as runway trackscollaborating with runway tracks positioned laterally on the toothedwheel (5).
 10. The mechanical transmission device as claimed in claim 1,characterized in that it comprises connecting means connecting the teeth(74) of the large-sized rack (73) of the control member (7) whichconsist of lateral cheeks (71, 72) stiffening the control member (7) andincreasing the bending strength of the teeth (74) of the large-sizedrack (73).
 11. The mechanical transmission device as claimed in claim 9,characterized in that the lateral cheeks (71, 72) can be used as runwaytracks collaborating with runway tracks positioned laterally on thetoothed wheel (5).
 12. The mechanical transmission device as claimed ineither one of claims 8 and 10, characterized in that it comprisestransmission (3) and control (7) members in which the large-sized racks(35, 73) comprise lateral cheeks (11, 12; 71, 72) which are produced bymachining and/or forging the bodies that make up said transmission andcontrol members.
 13. The mechanical transmission device as claimed ineither one of claims 8 and 10, characterized in that it comprisestransmission (3) and control (7) members in which the large-sized racks(35, 73) comprise lateral cheeks (11, 12; 71, 72) which are added ontoand fixed to bodies that make up said transmission and control members.14. The mechanical transmission device as claimed in claim 1,characterized in that it comprises a toothed wheel (5) exhibiting acutout (16) on its upper profile (59) so as to reduce the vertical bulkof the engine.
 15. The mechanical transmission device as claimed inclaim 1, characterized in that it comprises a toothed wheel (5) which ismade up of two toothed half-wheels (A, B) joined together.
 16. Themechanical transmission device as claimed in claim 15, characterized inthat it comprises a toothed wheel (5) which is made up of two toothedhalf-wheels (A, B) joined together exhibiting a cutout (16) on theirupper profile (59) so as to reduce the vertical bulk of the engine. 17.The mechanical transmission device as claimed in claim 1, characterizedin that it comprises lightening means for lightening the toothed wheel(5) which consist of at least one cavity (56).
 18. The mechanicaltransmission device as claimed in claim 1, characterized in that itcomprises a toothed wheel (5) and large-sized racks (35, 73) belongingto the transmission (3) and control (7) members, the teeth of which havedifferent widths.
 19. The mechanical transmission device as claimed inclaim 1, characterized in that it comprises a toothed wheel (5) andlarge-sized racks (35, 73) belonging to the transmission (3) and control(7) members, the teeth of which are of different dimensions.
 20. Themechanical transmission device as claimed in claim 1, characterized inthat it comprises a toothed wheel (5) and large-sized racks (35, 73)belonging to the transmission (3) and control (7) members, the teeth ofwhich have different profiles.
 21. The mechanical transmission device asclaimed in claim 1, characterized in that it comprises a toothed wheel(5) the teeth of which exhibit impressions (15) on their lateral faceswhich determine the stiffness of the edge of these teeth.
 22. Themechanical transmission device as claimed in claim 14, characterized inthat the toothed wheel (5) comprises a central cavity (53) of which theinternal faces exhibit guide surfaces (14) for the lateral guidance ofthe small end (60) of the connecting rod (6).
 23. The mechanicaltransmission device as claimed in claim 15, characterized in that itcomprises a toothed wheel (5) of which the internal faces of eachtoothed half-wheel (A, B) exhibit a guide surface (14) for the lateralguidance of the small end (60) of the connecting rod (6).
 24. Themechanical transmission device as claimed in either one of claims 14 and15, characterized in that a toothed wheel (5) exhibits a pivot axissituated in a bore (50) which is offset from the center of the pitchcircle of said toothed wheel (5) so as to obtain variations in thekinematics of the piston (2).
 25. The mechanical transmission device asclaimed in claim 1, characterized in that it comprises a transmissionmember (3) of which the threaded rod (31) exhibits, at its middle, ahole (33) which opens, on the one hand, above a runway track (36) of thetransmission member (3) and, on the other hand, into the threaded bore(24) formed in the supporting leg (22) of the piston (2).
 26. Themechanical transmission device as claimed in claim 1, characterized inthat the transmission (3) and control (7) members comprise runway tracks(36, 70) which are produced from the machining of the bodies that makeup said transmission and control members.
 27. The mechanicaltransmission device as claimed in claim 1, characterized in that itcomprises a transmission member (3) which is provided with lighteningmeans (13).
 28. The mechanical transmission device as claimed in claim1, characterized in that it comprises a rolling guide device (4) whichis made up of a synchronized roller (40) and of a small-sized rackdevice (41) secured to the engine block which collaborates with thesmall-sized rack (37) of the transmission member (3) so as, on the onehand, to guide the transmission member (3) on the engine block and, onthe other hand, to synchronize the vertical movement of said roller (40)with respect to the piston (2).
 29. The mechanical transmission deviceas claimed in claim 28, characterized in that the roller (40) consistsof a cylindrical body (42) comprising, at each end, pinions (44) and aguide groove (43) provided between the pinions.
 30. The mechanicaltransmission device as claimed in claim 28, characterized in that therack device (41) secured to the engine block is made up of a centralbody (45) forming a runway track (48) comprising, at its middle, avertical rib (49) which collaborates with the guide groove (43) of theroller (40) so as to provide lateral guidance therefor, and of racks(46) which are fixed onto the vertical edges (47) of said body (45) tocollaborate with the pinions (44) of the roller (40) so as tosynchronize the vertical movement of said roller with that of the piston(2).